1. Field of the Invention
This invention relates in general to earth boring bits and in particular to methods for threading bits after assembly and welding of the sections of which they are composed.
2. Background Information
Earth boring bits of the rolling cone type are often called "rock bits", even though some can drill geological formations softer than those commonly classified as rock. The rock bit has many of the features found in the original two cone bits of Howard R. Hughes, U.S. Pat. No. 930,759, Aug. 10, 1909.
The body of a typical, contemporary bit is constructed of three sections, assembled to engage across 120 degree, machined faces and welded to form an integral body unit. A cone is mounted, prior to welding, on a cantilevered bearing shaft that depends from each section.
In recent years attention has been focused on improvements to the welding and assembly procedures to improve the geometry, the dimensional accuracy and the integrity to design of the rock bit.
A method to improve the accuracy of the assembly and welding of the sections of a rock bit is show in U.S. patent of George W. Baur, U.S. Pat. No. 4,209,124, Rock Bit Assembly Method, June 24, 1980. Here, a shank positioner and a fixed ring are positioned with a fixture to hold the sections and assembled cones to minimize slippage of the sections during assembly and welding.
Another method to improve integrity to rock bit design parameters is disclosed in my previous U.S. Pat. No. 4,414,734, Triad For Rock Bit Assembly, Nov. 15, 1983. A specially shaped dowel called a triad is used to maintain the 120 degree faces of the sections in the intended relationship. This method further assures accuracy of alignment of the sections during assembly and welding.
Still, there is need for additional accuracy, especially in machining the threads on the shank of bits for accurate alignment of the sections and cones with the design centerline or rotational axis of the bit. Even though the sections of the bit are accurately aligned and welded, performance of the bit suffers if the threads of the shank are misaligned from the design centerline. The resulting eccentric rotation of the bit can produce accelerated wear of the cones and of the supporting bearings.